I think it is more driven by political agenda. A new T-18 train, with the name Vande -Bharat, will attract more eyes than "rolling stock of Shatabdi replaced by T-18". And I agree with you about similar possibility with T-20 in the future, although I'd be happy to be wrong.
Every RM introduces new train products, and vandalizes the idea of existing ones. Rajdhani and Shatabdi were untouched, parallel products were introduced as Duronto(copy of Rajdhani, excluding the caveat of Capital cities), but Rajdhani and Shatabdi were the high standards for short and long distance travels. Even when LHBs came, Rajdhani and Shatabdis were chosen initially for a reason. Sadly, it does not seem to be the case in future.